1. Field of the Invention
The present invention relates to a prism assembly, more specifically a prism assembly consisting of two or more prisms, at least one of which comprises one or more surfaces that are used for total internal reflection or partial spectral reflection of incoming light.
In particular the present invention relates to sealing from atmospheric contaminants the air gaps, specifically, the air gaps between prisms and the air gaps between the prisms and the light reflecting valves or more precisely digital micro-mirror devices (DMD), in a digital light processing (DLP) engine.
2. Discussion of the Relelated Art
In the past, cathode ray tube (CRT) projectors were used for most projection applications, but they have largely been supplanted, because they are large in size, difficult to calibrate, must be used in very dark surroundings, and have a high susceptibility to frequent breakdowns. Digital projector systems have become increasingly popular over the past decade, because of their ability to project high-quality images for applications, such as, for example, boardroom presentations, home theater systems, and large-scale stadium concerts. Liquid crystal display (LCD) is one technology used in digital projectors, but visible pixilation issues and larger size, compared to that of DLP, are negatives associated with LCD technology. DLP is a competing technology and is prized for its compactness and ability to deliver high levels of contrast and brightness.
Dr. Larry Hornbeck invented DLP technology at Texas Instruments in 1987. Digital projectors that use DLP technology contain a DLP light engine. DLP light engines have a lamp which shines onto a digital multi-mirror device (DMD) chip and an array of microscopic mirrors, each of which directs the light that hits it in either the ‘on-state’ or the ‘off-state.’ The ‘on-state’ light reaches the screen and produces a white (or color) pixel, while the ‘off-state’ light is absorbed inside the projector, which results in a dark pixel. For the best light-output and color saturation, advanced DLP systems use 3 DMD chips, one each for red, green, and blue. The 3 DMD chips are combined by means of a Philips prism, which is a combination of 5 prisms. The prism apparatus utilizes total internal reflection (TIR), which requires the existence of air gaps of 5 micrometers (μm) to 1 millimeter (mm) between the various prisms.
However, the prisms used in DLP projectors are highly sensitive to contamination. Digital projectors are often used in high-stress environments, such as concerts, where dust, smoke, cracked oil, and other contaminants are present in the atmosphere. The air gaps in the prism configuration are susceptible to accumulating these contaminants, which often results in the projection of visual artifacts, such as colored spots. Contaminations can block light or nullify a TIR condition. Cleaning the prisms from the contaminants is impractical, because of the high cost and difficulty of cleaning. What is needed is a means of protecting the prisms and DMD chips in a DLP light engine from atmospheric contamination.
Currently, a popular procedure for protecting the light engine from contaminants is to enclose and protect the DLP engine in a metal box. An example of such an apparatus is seen in reference to U.S. Pat. No. 6,350,033, entitled “Projector.” The '033 patent details a structure that encloses an optical path from an electro-optical apparatus to a projection lens by means of an attached air circulation path, which is disposed in a flow path for cooling air. Thus, the air circulation path is able to cool the air enclosed in the structure, while preventing dust or oil contamination. However, the '033 patent fails to provide a means of dissipating a sufficient amount of heat for normal projector operation when the light illuminates greater than 5,000 lumens. If the heat is not sufficiently dissipated, sensitive optics may warp, which causes visual artifacts to appear on the projected images and other negative effects. Excessive heat may also damage the DMD chips. Therefore, what is further needed is a means of permitting sufficient heat dissipation from the prisms and DMD chips in a DLP light engine.
To project a visual-artifact-free image, all TIR conditions must be preserved, and light must be allowed to pass through all the necessary areas of the prisms in the DLP light engine. Thus, what is further needed is a means of fulfilling all TIR conditions and allowing light to pass through all necessary areas in the prisms and DMD chips in a DLP light engine.